Have you tried to get "extra" cylinders from Ruger?
The last I talked to them (2013), they would not sell a blank cylinder, because they want to fit the cylinder to the frame.
Do you have a machinist who can mill out a cylinder from stock?
There used to be one in Omaha who would make 5-shot cylinders for the big-ass cartridges like .458, .460, etc.
I still have the dream of getting a 12" K-Blackhawk in .256 Mag.
Yes, but Ruger customer service says 'not available separately', and also 8-shot cylinders are 'aftermarket' anyway at this time.
There is the "Problem" that IF I send my .25ACP Ruger back to Ruger, FIRST they Will restore it to FACTORY Configuration before it could have any other work done on it. their Policy is NOT to repair 'modified' firearms unless they are restored to Factory configuration first.
Bummer, but presently I have a "work around" so i am currently'good to go' with this project.
Chev. William #736